Literacy was not exactly rampant in this era, and keeping a history was generally the job of the skalds and scops (look 'em up). Most of this history was probably not committed to paper (Office Depots being scarce back then), but set in the form of poetry (see Homer's Iliad for example—or more to the point, Beowulf). Rhyme and meter, and for that matter, tunes, are good mnemonic devices. The skalds and scops (later, bards and minstrels) would compose these stories, then recite or chant them. And like many things in the oral tradition (such as fish-stories) sometimes they would "dramatize" things a bit, or they would grow in the repetition, so the heroes (often the king or chief who employed the scop) often took on very heroic proportions. It may very well be that some of the ballads that appear in Child are later versions of these "histories" and heroic stories.   

If I were trying to come up with something that sounded like it was contemporary with King Arthur, I think I would take some of the earliest of the Child ballads or similar works and try to rewrite them into something that sounded even earlier. Or write something original in that style. Now, I'm not exactly sure how I would do that. . . .

Just a thought or two.

Don Firth
(King of the parenthetical phrase)